Park visitors walk past the dredging process that is cleaning up years of pollution at Mountain Lake.

Park visitors walk past the dredging process that is cleaning up years of pollution at Mountain Lake.

Photo: Michael Macor, The Chronicle

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Unfiltered lake water (left) is next to water with polymers that separate the water and sediment, and clean water ready to go back to the lake.

Unfiltered lake water (left) is next to water with polymers that separate the water and sediment, and clean water ready to go back to the lake.

Photo: Michael Macor, The Chronicle

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The 230 foot long bladders containing the sediment being pumped from Mountain Lake in the Richmond District of in San Francisco, Calif. on Tues. May 21, 2013. The water and sediment is then separated and filtered and water is then returned clean back to the lake.The Mountain Lake Enhancement Project is a multi-phase effort to improve the health of Mountain Lake by removing sediment to deepen the lake and improve water quality, expanding wildlife habitat through landscape restoration and enhancing the visitor experience. less

The 230 foot long bladders containing the sediment being pumped from Mountain Lake in the Richmond District of in San Francisco, Calif. on Tues. May 21, 2013. The water and sediment is then separated and ... more

Photo: Michael Macor, The Chronicle

Mountain Lake's huge cleanup effort

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Toxic sludge is being pumped out of San Francisco's Mountain Lake this week in an operation that officials hope will transform what has for years been a filthy pond full of alien species into a clean, healthy natural preserve.

The dredging of accumulated lead, oil and other contaminants from the bottom of the spring-fed lake will be done in two phases and is expected to take at least five months. It is part of an innovative effort to restore the ancient lake ecosystem and resurrect as much of the area's ecological history as possible.

The muck raking operation will remove 15,600 cubic yards of sediment saturated with lead that leaked from cars on adjacent Park Presidio Boulevard during the leaded gasoline era. It will also increase the depth, reduce the temperature of the lake and, when all the non-native fish and turtles are removed, allow for the reintroduction of native species.

"The water should be clear when we are done because we are taking out a lot of the accumulated organics at the bottom of the lake," said Eileen Fanelli, the environmental remediation program manager for the Presidio Trust, which is in charge of the cleanup.

The effort by the trust, the National Park Service and the Golden Gate National Parks Conservancy is an experiment in urban landscape engineering the likes of which the city has never before seen.

The dredging required construction of an entire water treatment plant. The sediment is being sucked up through pipes, where it is injected with chemicals that separate the water from the silt and pumped into four humongous 230-foot-long specialized containers near the lake.

The sediment is captured inside the porous sack-like containers, which look like giant black sand bags. The water leaks out of the sacks and flows downward into a pool, where it is sucked into six 20,000-gallon filtering tanks. The dirt-colored water is then pushed through sand filters, pumped into tanks of clay and carbon and then discharged back into the lake free of sediment and fully in compliance with Regional Water Quality Control Board criteria.

The dredging is the most expensive project in the restoration plan that is expected to cost between $9 million and $12 million - $5.5 million is paid in mitigation by Caltrans, which took responsibility for the leaded gasoline leakage from Park Presidio.

Larger habitat

In addition to the dredging, 1,800 cubic yards of soil will be removed from the edges of the 4-acre lake in an attempt to enlarge habitat. Some 6,000 cubic yards now covered by a tangle of eucalyptus and vegetation will also be cleared in an attempt to restore marshlands in what once was the east arm of the historic body of water.

"We cleared about a half acre of tule and between 30 to 50 logs from the lake before we started dredging," said Genevieve Coyle, the environmental remediation project manager for the trust, which has also planted pine trees, native grasses and cut down groves of litter-prone eucalyptus trees.

Mountain Lake is one of the few remaining natural lakes in San Francisco and was once a valuable source of drinking water for the Ohlone Indians and European settlers.

Many of the native frogs and turtles were cleaned out during the Gold Rush so restaurants could serve their tasty legs or put them in soup. Originally 30 feet deep, the lake shrank by 40 percent when Park Presidio Boulevard was built in 1939.

Dozens of invasive species were introduced, including bullfrogs, large-mouth bass, non-native turtles and dozens of pet goldfish. Lead from car emissions combined with zinc, petroleum hydrocarbons from motor oil and pesticides flowing in from an adjacent golf course to form a toxic stew.

The native fish and frogs have long since died out and by the time dredging started, the lake itself was little more than a sludge-filled pool, festering with leeches, midge fly larvae and other unsavory creatures that like murky water.

The restoration actually began last July when more than 10,000 fish were removed. That included 42 nonnative turtles, two or three dozen adult carp and large bass, some weighing as much as 15 pounds.

"Two sturgeons were caught last year. One was about 4 feet long," said Terri Thomas, the trust's director of conservation, stewardship and research. Nobody knows how the sturgeon got there.

Removing species

The idea is to eventually reintroduce native aquatic plants, fish, turtles and mussels, but the alien species that remain in the lake must first be removed, Thomas said. The trust is considering the introduction of koi herpes, genetically modified carp or even poisoning the lake, but nobody has yet figured out how to end what is apparently a neighborhood tradition of dumping unwanted aquarium pets.

"We're still hoping to get rid of all of (the non-natives), but we really won't know what to do until the dredging is done," said Thomas, who has overseen the placement of signs urging - pleading with - lake visitors to stop liberating their pet reptiles and fish.